棒状ZnO基稀磁半导体的制备及性能研究

【作者】 杭联茂；

【导师】 张志勇；

【作者基本信息】 中国科学院研究生院（西安光学精密机械研究所） ， 物理电子学， 2011， 博士

【摘要】 氧化锌(ZnO)是一种具有3.37eV的禁带宽度、高达60meV的激子束缚能、高的化学稳定性以及优良的压电、热电和光电特性的新型宽禁带Ⅱ-Ⅵ族化合物半导体材料。尤为重要的是,ZnO基稀磁性半导体材料由于可能具有高于室温的居里温度,大的磁性离子固溶度以及对可见光透明等特点,因此有希望成为集优异的磁、光、电等特性为一体的新型多功能半导体材料,在未来的自旋电子学器件中具有广阔的应用前景。然而由于目前获得的ZnO基稀磁半导体材料存在着实验的重复性差、铁磁性不稳定以及铁磁性来源没有统一的理论解释等问题,因此,开展ZnO基稀磁半导体的实验和理论研究具有重要的现实意义。本文首先采用水热法,以醋酸锌和氢氧化钠为原料,制备了棒状ZnO,并分析了棒状ZnO的形成机理。同时研究了表面活性剂PAM对棒状ZnO结构和性能的影响。结果表明,PAM对ZnO的形貌没有明显的影响,但可以改变样品的化学计量比,起到调节主导缺陷类型和数目的作用。棒状ZnO晶体的紫色和蓝色发射随着PAM浓度的增加先增加后减小,而绿色发射随着随着PAM浓度的增加而增加。在PAM浓度分别为0、0.00005%、0.0003%时的棒状ZnO样品均呈现出顺磁性的特点。而在PAM浓度为0.0001%时的棒状ZnO样品呈现出室温铁磁性,其中的Zn空位缺陷占主导地位,被认为是产生铁磁性的主要原因,磁矩的主要贡献来自于紧邻Zn空位的O原子的2p未成对电子。其次,分别以乙酸铬、乙酸钴和乙酸镍为掺杂剂,对棒状ZnO进行了掺杂,并对其磁光性能进行了研究。掺杂剂的引入对样品的生长特性产生一定影响,虽然ZnO晶体仍然沿着[001]方向择优生长,但是样品变得粗大,而且均匀性变差,同时棒状ZnO晶体的本征缺陷增加,结晶质量下降。Cr掺杂棒状ZnO晶体的所有可见光发射及Co掺杂棒状ZnO晶体的紫色和蓝色发射随着掺杂浓度的增加先增加后减小；Ni掺杂棒状ZnO晶体的可见光发射随着掺杂浓度的增加而增加；Co掺杂棒状ZnO晶体的绿光发射随着掺杂浓度的增加而减小。掺杂浓度为0.5%Cr、Co、Ni掺杂棒状ZnO样品,由于掺杂浓度过小,均呈现出顺磁性的特征。1%Cr、Co、Ni掺杂棒状ZnO样品均呈现出室温铁磁性。3%Cr掺杂棒状ZnO样品,由于掺杂浓度增加,近距离Cr离子间的反铁磁作用使得样品呈现顺磁性。3%Co掺杂棒状ZnO样品由于反铁磁性第二相C0304的产生,使得样品的铁磁性相比于1%Co棒状ZnO样品有所减弱。3%Ni掺杂棒状ZnO样品由于反铁磁第二相Ni(OH)2的产生,使得样品呈现反铁磁性的特征。最后采用基于密度泛函理论框架下的第一性原理计算方法,系统研究了ZnO体材料和纳米线以及Cr、Co、Ni分别掺杂的ZnO体材料和纳米线的电子结构和磁学属性,并对ZnO体相材料和纳米线材料以及Co掺杂ZnO体相材料和纳米线材料的光学性质进行了研究。结果表明,ZnO体相材料和纳米线材料均不具有磁性,由于量子限制效应,使得ZnO纳米线材料的禁带宽度相比体相材料有所展宽。Co掺杂ZnO体相材料和Co替代ZnO纳米线体内Zn原子的纳米线材料均呈现出了铁磁性,而且纳米线的磁性相比于体材料有一定程度的放大,其磁性来源于Co3d和02p的交换作用使得能级发生劈裂,产生自旋极化。而Co替代ZnO纳米线表面的Zn原子的体系均不具有磁性。另外,Cr、Ni掺杂ZnO体相材料及纳米线材料也均不呈现磁性。从而说明缺陷对样品呈现的宏观铁磁性起着至关重要的作用。更多还原

【Abstract】 ZnO is an attractive semiconductor with a direct wide-band gap (3.37eV) and large exciton binding energy (60meV) at room temperature and high chemical stability, excellent piezoelectric, pyroelectric and optoelectronic properties. More importantly, diluted magnetic semiconductors (DMSs) based on ZnO become a good candidate for potential spintronics integrating the excellent optical, electronic and magnetic properties because of their ferromagnetic properties at or above room temperature predicted by theoretical studies, large solubility of magnetic ions and being transparent to visible light. However, up to now, the ferromagnetism of the obtained DMSs based on ZnO is not stable. Experimental reproducibility is poor and the origin of ferromagnetism has no unified theory of interpretation. So it is of great practical significance to study DMSs based on ZnO through experiment and theory.Firstly, Rod-like ZnO crystals were synthesized by hydrothermal method employing zinc acetate dihydrate [Zn(CHsCOO)2·2H2O] and sodium hydroxide (NaOH) as the starting reactants and their growth mechanisms were tentatively elucidated. Moreover the effect of surfactant PAM on the structures and properties of rod-like ZnO crystals were also investigated. The results show that the PAM can hardly influence the morphologies of Rod-like ZnO crystals, but it can alter the stoichiometric ratio and adjust the type and number of the dominant intrinsic defects. The intensities of the violet and blue emissions of the samples increase at first, and then decrease, and the green emission increase with the increase of PAM concentration. All the samples synthesized with the PAM concentration of0%,0.00005%and0.0003%show paramagnetism. When the PAM concentration is0.0001%, the rod-like ZnO crystals exhibit ferromagnetism at room temperature, in which zinc vacancies are the main defects and considered to be the origin of ferromagnetism.Secondly, the structural, magnetic and optical properties of rod-like ZnO doped with Cr, Co and Ni were studied using chromium acetate [Cr(CH3COO)3], cobalt acetate tetrahydrate [Co(CH3COO)2·4H2O] and nickel acetate tetrahydrate [Ni(CH3COO)2·4H2O] as dopants, respectively. Althougt all the rods grow along the preferred direction of [001], they grow larger, the nonuniformity become more obvious and the instrinc defects increase with the increase of doping concentration. The intensities of the visible emission of Cr-doped ZnO and the violet and blue emissions of Co-doped ZnO increase at first, and then decrease with the increase of doping concentration. The intensities of the visible emission of Ni-doped ZnO increase and the green emission of Co-doped ZnO decrease with the increase of doping concentration. All the samples of ZnO doped with0.5%Cr, Co or Ni show paramagnetism. The samples of ZnO doped with1%Cr, Co or Ni exhibit ferromagnetism at room temperature.3%Cr-doped ZnO exhibits paramagnetism because of the antiferromagnetic interaction between Cr ions. The ferromagnetism of3%Co-doped ZnO become weaker comparing with1%Co-doped ZnO due to the antiferromagnetic secondary phase of CO3O4.3%Ni-doped ZnO exhibits antiferromagnetism because antiferromagnetic secondary phase of Ni(OH)2come into being during hydrothermal process.Lastly, the electronic structures, magnetic and optical properties of bulk ZnO and ZnO nanowires undoped or doped with Cr, Co or Ni were investigated by the first-principles calculation based on the density functional theory. The results show that the undoped bulk ZnO and ZnO nanowires exhibit no magnetism and the band gap of ZnO nanowires is wider than that of bulk ZnO because of quantum confinement effect. The Co-doped bulk ZnO exhibits ferromagnetism. The Co-doped ZnO nanowires also show ferromagnetism when the Co atoms substitute for Zn sites in the body of nanowires and its magnetization become larger than that of Co-doped bulk ZnO. Their ferromagnetism originates from the coupling interaction of the electronics between Co3d and O2p. However, the nanowires show no magnetism when the Co atoms substitute for Zn sites on the surface of nanowires. The bulk ZnO and ZnO nanowires doped with Cr or Ni all exhibit no magnetism whenever the Cr or Ni substitutes for Zn in the body or on the surface of nanowires. The defects play an important role in the ferromagnetism of Cr and Ni doped ZnO samples.更多还原